Communication system

ABSTRACT

Provided is a technology capable of securing satisfactory communication quality. A communication system includes a user equipment, and a base station configured to be connected to the user equipment to perform radio communication with the user equipment. The user equipment performs radio communication with a beam. When the user equipment detects a beam disappearance state being a state incapable of maintaining communication quality with the base station, the user equipment transmits a notification of the beam disappearance state with a beam having a wider half width than a half width before detection of the beam disappearance state.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of and claims the benefit of priorityunder 35 U.S.C. § 120 from U.S. application Ser. No. 16/992,217 filedAug. 13, 2020, which is a division of U.S. application Ser. No.16/484,899 filed Aug. 9, 2019 (now U.S. Pat. No. 10,785,661 issued Sep.22, 2020), the entire contents of which are incorporated herein byreference. U.S. application Ser. No. 16/484,899 is a National Stage ofPCT/JP2018/007132 filed Feb. 27, 2018, which claims the benefit ofpriority under 35 U.S.C. § 119 from Japanese Application No. 2017-053919filed Mar. 21, 2017.

TECHNICAL FIELD

The present invention relates to a communication system.

BACKGROUND ART

The 3rd generation partnership project (3GPP), the standard organizationregarding the mobile communication system, is studying communicationsystems referred to as long term evolution (LTE) regarding radiosections and system architecture evolution (SAE) regarding the overallsystem configuration including a core network and a radio access networkwhich is hereinafter collectively referred to a network as well (forexample, see Non-Patent Documents 1 to 4). This communication system isalso referred to as 3.9 generation (3.9 G) system.

As the access scheme of the LTE, orthogonal frequency divisionmultiplexing (OFDM) is used in a downlink direction and single carrierfrequency division multiple access (SC-FDMA) is used in an uplinkdirection. Further, differently from the wideband code division multipleaccess (W-CDMA), circuit switching is not provided but a packetcommunication system is only provided in the LTE.

The decisions taken in 3GPP regarding the frame configuration in the LTEsystem described in Non-Patent Document 1 (Chapter 5) are described withreference to FIG. 1 . FIG. 1 is a diagram illustrating the configurationof a radio frame used in the LTE communication system. With reference toFIG. 1 , one radio frame is 10 ms. The radio frame is divided into tenequally sized subframes. The subframe is divided into two equally sizedslots. The first and sixth subframes contain a downlink synchronizationsignal per radio frame. The synchronization signals are classified intoa primary synchronization signal (P-SS) and a secondary synchronizationsignal (S-SS).

Non-Patent Document 1 (Chapter 5) describes the decisions by 3GPPregarding the channel configuration in the LTE system. It is assumedthat the same channel configuration is used in a closed subscriber group(CSG) cell as that of a non-CSG cell.

A physical broadcast channel (PBCH) is a channel for downlinktransmission from a base station device (hereinafter may be simplyreferred to as a “base station”) to a communication terminal device(hereinafter may be simply referred to as a “communication terminal”)such as a user equipment device (hereinafter may be simply referred toas a “user equipment”). A BCH transport block is mapped to foursubframes within a 40 ms interval. There is no explicit signalingindicating 40 ms timing.

A physical control format indicator channel (PCFICH) is a channel fordownlink transmission from a base station to a communication terminal.The PCFICH notifies the number of orthogonal frequency divisionmultiplexing (OFDM) symbols used for PDCCHs from the base station to thecommunication terminal. The PCFICH is transmitted per subframe.

A physical downlink control channel (PDCCH) is a channel for downlinktransmission from a base station to a communication terminal. The PDCCHnotifies of the resource allocation information for downlink sharedchannel (DL-SCH) being one of the transport channels described below,resource allocation information for a paging channel (PCH) being one ofthe transport channels described below, and hybrid automatic repeatrequest (HARQ) information related to DL-SCH. The PDCCH carries anuplink scheduling grant. The PDCCH carries acknowledgement(Ack)/negative acknowledgement (Nack) that is a response signal touplink transmission. The PDCCH is referred to as an L1/L2 control signalas well.

A physical downlink shared channel (PDSCH) is a channel for downlinktransmission from a base station to a communication terminal. A downlinkshared channel (DL-SCH) that is a transport channel and a PCH that is atransport channel are mapped to the PDSCH.

A physical multicast channel (PMCH) is a channel for downlinktransmission from a base station to a communication terminal. Amulticast channel (MCH) that is a transport channel is mapped to thePMCH.

A physical uplink control channel (PUCCH) is a channel for uplinktransmission from a communication terminal to a base station. The PUCCHcarries Ack/Nack that is a response signal to downlink transmission. ThePUCCH carries a channel quality indicator (CQI) report. The CQI isquality information indicating the quality of received data or channelquality. In addition, the PUCCH carries a scheduling request (SR).

A physical uplink shared channel (PUSCH) is a channel for uplinktransmission from a communication terminal to a base station. An uplinkshared channel (UL-SCH) that is one of the transport channels is mappedto the PUSCH.

A physical hybrid ARQ indicator channel (PHICH) is a channel fordownlink transmission from a base station to a communication terminal.The PHICH carries Ack/Nack that is a response signal to uplinktransmission. A physical random access channel (PRACH) is a channel foruplink transmission from the communication terminal to the base station.The PRACH carries a random access preamble.

A downlink reference signal (RS) is a known symbol in the LTEcommunication system. The following five types of downlink referencesignals are defined as: a cell-specific reference signal (CRS), an MBSFNreference signal, a data demodulation reference signal (DM-RS) being auser equipment-specific reference signal (UE-specific reference signal),a positioning reference signal (PRS), and a channel state informationreference signal (CSI-RS). The physical layer measurement objects of acommunication terminal include reference signal received powers (RSRPs).

The transport channels described in Non-Patent Document 1 (Chapter 5)are described. A broadcast channel (BCH) among the downlink transportchannels is broadcast to the entire coverage of a base station (cell).The BCH is mapped to the physical broadcast channel (PBCH).

Retransmission control according to a hybrid ARQ (HARQ) is applied to adownlink shared channel (DL-SCH). The DL-SCH can be broadcast to theentire coverage of the base station (cell). The DL-SCH supports dynamicor semi-static resource allocation. The semi-static resource allocationis also referred to as persistent scheduling. The DL-SCH supportsdiscontinuous reception (DRX) of a communication terminal for enablingthe communication terminal to save power. The DL-SCH is mapped to thephysical downlink shared channel (PDSCH).

The paging channel (PCH) supports DRX of the communication terminal forenabling the communication terminal to save power. The PCH is requiredto be broadcast to the entire coverage of the base station (cell). ThePCH is mapped to physical resources such as the physical downlink sharedchannel (PDSCH) that can be used dynamically for traffic.

The multicast channel (MCH) is used for broadcasting to the entirecoverage of the base station (cell). The MCH supports SFN combining ofmultimedia broadcast multicast service (MBMS) services (MTCH and MCCH)in multi-cell transmission. The MCH supports semi-static resourceallocation. The MCH is mapped to the PMCH.

Retransmission control according to a hybrid ARQ (HARQ) is applied to anuplink shared channel (UL-SCH) among the uplink transport channels. TheUL-SCH supports dynamic or semi-static resource allocation. The UL-SCHis mapped to the physical uplink shared channel (PUSCH).

A random access channel (RACH) is limited to control information. TheRACH involves a collision risk. The RACH is mapped to the physicalrandom access channel (PRACH).

The HARQ is described. The HARQ is the technique for improving thecommunication quality of a channel by combination of automatic repeatrequest (ARQ) and error correction (forward error correction). The HARQis advantageous in that error correction functions effectively byretransmission even for a channel whose communication quality changes.In particular, it is also possible to achieve further qualityimprovement in retransmission through combination of the receptionresults of the first transmission and the reception results of theretransmission.

An example of the retransmission method is described. If the receiverfails to successfully decode the received data, in other words, if acyclic redundancy check (CRC) error occurs (CRC=NG), the receivertransmits “Nack” to the transmitter. The transmitter that has received“Nack” retransmits the data. If the receiver successfully decodes thereceived data, in other words, if a CRC error does not occur (CRC=OK),the receiver transmits “AcK” to the transmitter. The transmitter thathas received “Ack” transmits the next data.

The logical channels described in Non-Patent Document 1 (Chapter 6) aredescribed. A broadcast control channel (BCCH) is a downlink channel forbroadcast system control information. The BCCH that is a logical channelis mapped to the broadcast channel (BCH) or downlink shared channel(DL-SCH) that is a transport channel.

A paging control channel (PCCH) is a downlink channel for transmittingpaging information and system information change notifications. The PCCHis used when the network does not know the cell location of acommunication terminal. The PCCH that is a logical channel is mapped tothe paging channel (PCH) that is a transport channel.

A common control channel (CCCH) is a channel for transmission controlinformation between communication terminals and a base station. The CCCHis used in a case where the communication terminals have no RRCconnection with the network.

In the downlink direction, the CCCH is mapped to the downlink sharedchannel (DL-SCH) that is a transport channel. In the uplink direction,the CCCH is mapped to the uplink shared channel (UL-SCH) that is atransport channel.

A multicast control channel (MCCH) is a downlink channel forpoint-to-multipoint transmission. The MCCH is used for transmission ofMBMS control information for one or several MTCHs from a network to acommunication terminal. The MCCH is used only by a communicationterminal during reception of the MBMS. The MCCH is mapped to themulticast channel (MCH) that is a transport channel.

A dedicated control channel (DCCH) is a channel that transmits dedicatedcontrol information between a communication terminal and a network on apoint-to-point basis. The DCCH is used when the communication terminalhas an RRC connection. The DCCH is mapped to the uplink shared channel(UL-SCH) in uplink and mapped to the downlink shared channel (DL-SCH) indownlink.

A dedicated traffic channel (DTCH) is a point-to-point communicationchannel for transmission of user information to a dedicatedcommunication terminal. The DTCH exists in uplink as well as downlink.The DTCH is mapped to the uplink shared channel (UL-SCH) in uplink andmapped to the downlink shared channel (DL-SCH) in downlink.

A multicast traffic channel (MTCH) is a downlink channel for trafficdata transmission from a network to a communication terminal. The MTCHis a channel used only by a communication terminal during reception ofthe MBMS. The MTCH is mapped to the multicast channel (MCH).

CGI represents a cell global identifier. ECGI represents an E-UTRAN cellglobal identifier. A closed subscriber group (CSG) cell is introducedinto the LTE, and the long term evolution advanced (LTE-A) and universalmobile telecommunication system (UMTS) described below.

The closed subscriber group (CSG) cell is a cell in which subscriberswho are allowed to use are specified by an operator (hereinafter, alsoreferred to as a “cell for specific subscribers”). The specifiedsubscribers are allowed to access one or more cells of a public landmobile network (PLMN). One or more cells to which the specifiedsubscribers are allowed access are referred to as “CSG cell(s)”. Notethat access is limited in the PLMN.

The CSG cell is part of the PLMN that broadcasts a specific CSG identity(CSG ID) and broadcasts “TRUE” in a CSG indication. The authorizedmembers of the subscriber group who have registered in advance accessthe CSG cells using the CSG ID that is the access permissioninformation.

The CSG ID is broadcast by the CSG cell or cells. A plurality of CSG IDsexist in the LTE communication system. The CSG IDs are used by userequipments (UEs) for making access from CSG-related members easier.

The locations of communication terminals are tracked based on an areacomposed of one or more cells. The locations are tracked for enablingtracking the locations of communication terminals and callingcommunication terminals, in other words, incoming calling tocommunication terminals even in an idle state. An area for trackinglocations of communication terminals is referred to as a tracking area.

In 3GPP, base stations referred to as Home-NodeB (Home-NB; HNB) andHome-eNodeB (Home-eNB; HeNB) are studied. HNB/HeNB is a base stationfor, for example, household, corporation, or commercial access servicein UTRAN/E-UTRAN. Non-Patent Document 2 discloses three different modesof the access to the HeNB and HNB. Specifically, an open access mode, aclosed access mode, and a hybrid access mode are disclosed.

Further, specifications of long term evolution advanced (LTE-A) arepursued as Release 10 in 3GPP (see Non-Patent Documents 3 and 4). TheLTE-A is based on the LTE radio communication system and is configuredby adding several new techniques to the system.

Carrier aggregation (CA) is studied for the LTE-A system in which two ormore component carriers (CCs) are aggregated to support widertransmission bandwidths up to 100 MHz. Non-Patent Document 1 describesthe CA.

In a case where CA is configured, a user equipment has a single RRCconnection with a network (NW). In RRC connection, one serving cellprovides NAS mobility information and security input. This cell isreferred to as a primary cell (PCell). In downlink, a carriercorresponding to PCell is a downlink primary component carrier (DL PCC).In uplink, a carrier corresponding to PCell is an uplink primarycomponent carrier (UL PCC).

A secondary cell (SCell) is configured to form a serving cell group witha PCell, in accordance with the user equipment capability. In downlink,a carrier corresponding to SCell is a downlink secondary componentcarrier (DL SCC). In uplink, a carrier corresponding to SCell is anuplink secondary component carrier (UL SCC).

A serving cell group of one PCell and one or more SCells is configuredfor one user equipment.

The new techniques in the LTE-A include the technique of supportingwider bands (wider bandwidth extension) and the coordinated multiplepoint transmission and reception (CoMP) technique. The CoMP studied forLTE-A in 3GPP is described in Non-Patent Document 1.

The traffic flow of a mobile network is on the rise, and thecommunication rate is also increasing. It is expected that thecommunication rate is further increased when the operations of the LTEand the LTE-A are fully initiated.

Furthermore, the use of small eNBs (hereinafter also referred to as“small-scale base station devices”) configuring small cells is studiedin 3GPP to satisfy tremendous traffic in the future. In an exampletechnique under study, a large number of small eNBs is installed toconfigure a large number of small cells, which increases spectralefficiency and communication capacity. The specific techniques includedual connectivity (abbreviated as DC) with which a user equipmentcommunicates with two eNBs through connection thereto. Non-PatentDocument 1 describes the DC.

For eNBs that perform dual connectivity (DC), one may be referred to asa master eNB (abbreviated as MeNB), and the other may be referred to asa secondary eNB (abbreviated as SeNB).

For increasingly enhanced mobile communications, the fifth generation(hereinafter also referred to as “5G”) radio access system is studiedwhose service is aimed to be launched in 2020 and afterward. Forexample, in the Europe, an organization named METIS summarizes therequirements for 5G (see Non-Patent Document 5).

The requirements in the 5G radio access system show that a systemcapacity shall be 1000 times as high as, a data transmission rate shallbe 100 times as high as, a data latency shall be one tenth ( 1/10) aslow as, and simultaneously connected communication terminals 100 timesas many as those of the LTE system, to further reduce the powerconsumption and device cost.

To satisfy such requirements, increasing the transmission capacity ofdata using broadband frequencies, and increasing the transmission rateof data through increase in the spectral efficiency are being studied.To realize these, the techniques enabling the spatial multiplexing suchas the Multiple Input Multiple Output (MIMO) and the beamforming using amulti-element antenna are being studied.

The MIMO is continuously studied also in LTE-A. From Release 13, fulldimension (FD)-MIMO is studied as the extension of the MIMO, which usestwo-dimensional antenna array. Non-Patent Document 6 describes theFD-MIMO.

It is studied that the 5G radio access system will be installedconcurrently with the LTE system in the initial period of the launch ofits service, which is scheduled in 2020. The following configuration isconsidered. Specifically, an LTE base station and a 5G base station areconnected in a DC configuration, and the LTE base station is regarded asan MeNB and the 5G base station as an SeNB. C-plane data is processed inthe LTE base station having a large cell range, and U-plane is processedin the LTE base station and the 5G base station.

PRIOR ART DOCUMENTS Non-Patent Documents

-   Non-Patent Document 1: 3GPP TS 36.300 V13.0.0-   Non-Patent Document 2: 3GPP S1-083461-   Non-Patent Document 3: 3GPP TR 36.814 V9.0.0-   Non-Patent Document 4: 3GPP TR 36.912 V10.0.0-   Non-Patent Document 5: “Scenarios, requirements and KPIs for 5G    mobile and wireless system”, ICT-317669-METIS/D1.1-   Non-Patent Document 6: 3GPP TR 36.897 V13.0.0

SUMMARY Problem to be Solved by the Invention

The 5G system requires large-volume communication, and therefore also inthe user equipment, forming beams with a super multi-element antennahaving more than eight elements has been studied.

A method of forming beams with two stages in the base station to reducea processing volume has been known. This is a method in which aplurality of basic beams having narrowed directivity are formed in afirst stage, and SN enhancement or null formation is performed by usingthe first-stage beams in a second stage. The following two types ofmethods have been studied. One method is a hybrid method in which beamsare formed in an analog manner in the first stage, and beams are formeddigitally in the second stage. According to the hybrid method,processing of a digital unit can be reduced. The other method is afull-digital method in which beams are digitally formed also in thefirst stage by reducing the number of first-stage beams to the numberthat can be processed. According to the full-digital method, analogdispersion of frequency characteristics or the like can be reduced.

However, there has been no specific and clear instances as to formationand control of beams in the user equipment. Particularly, even when theuser equipment performs time division duplex (TDD) communication withthe base station, and the base station performs processing such asprecoding by using reciprocity of uplink and downlink channels totransmit a signal, the base station cannot take interference in the userequipment into consideration, and thus optimal precoding cannot beperformed. Therefore, there is a problem in that interference occurs inthe user equipment due to a signal with another base station.

The present invention has an object to solve the above-mentioned problemand provide a technology capable of securing satisfactory communicationquality.

For the sake of such an object, for example, the present inventionprovides a communication procedure necessary between a user equipmentand a base station for specifically forming beams in the user equipment,and can thereby secure satisfactory communication quality.

Further, for example, the present invention provides a technology offorming first-stage basic beams of the user equipment with beams that donot interfere with another base station different from a base station tobe communicated with, and can thereby secure satisfactory communicationquality even when an opposing base station being a communicationcounterpart does not take interference of another base station intoconsideration.

Means to Solve the Problem

A first communication system of the present invention includes a userequipment, and a base station configured to be connected to the userequipment to perform radio communication with the user equipment. Theuser equipment performs radio communication with a beam. When the userequipment detects a beam disappearance state being a state incapable ofmaintaining communication quality with the base station, the userequipment transmits a notification of the beam disappearance state witha beam having a wider half width than a half width before detection ofthe beam disappearance state.

A second communication system of the present invention includes a userequipment, and a base station configured to be connected to the userequipment to perform radio communication with the user equipment. Theuser equipment performs radio communication with a beam. When the userequipment detects a beam disappearance state being a state incapable ofmaintaining communication quality with a first base station, the userequipment transmits a notification of the beam disappearance state to asecond base station configuring dual connectivity with the first basestation. When the second base station receives the notification of thebeam disappearance state, the second base station gives a command to thefirst base station to perform beam re-detection processing with the userequipment.

A third communication system of the present invention includes a userequipment, and a base station configured to be connected to the userequipment to perform radio communication with the user equipment. Theuser equipment performs radio communication with a two-stage beamformingmethod by using a multi-element antenna. The user equipment transmitsinformation for identifying an attribute of each first-stage beam to thebase station.

A fourth communication system of the present invention includes a userequipment, and a base station configured to be connected to the userequipment to perform radio communication with the user equipment. Whenthe user equipment communicates with a first base station while notcommunicating with a second base station, the user equipment measures adegree that the second base station interferes with a transmissionsignal from the first base station, and transmits a measurement resultto the first base station. The first base station changes transmissionpower of a signal to be transmitted to the user equipment, based on areceived measurement result.

A fifth communication system of the present invention includes a userequipment, and a base station configured to be connected to the userequipment to perform radio communication with the user equipment. Whenthe user equipment communicates with a first base station while notcommunicating with a second base station, the first base station adjustsa communication condition for interference suppression between datatransmission from the first base station to the user equipment and datatransmission performed by the second base station, and requests thesecond base station to perform data transmission on the adjustedcommunication condition.

A sixth communication system of the present invention includes a userequipment, and a base station configured to be connected to the userequipment to perform radio communication with the user equipment. Theuser equipment performs radio communication with a two-stage beamformingmethod by using a multi-element antenna. When the user equipmentcommunicates with a first base station while not communicating with asecond base station, the user equipment forms, as first-stage beams, afirst beam having a main beam directed to the first base station and anull directed to the second base station, and at least one second beamhaving a null directed to the second base station and having directivitydifferent from directivity of the first beam.

A seventh communication system of the present invention includes a userequipment, and a base station configured to be connected to the userequipment to perform radio communication with the user equipment. Theuser equipment performs radio communication with a two-stage beamformingmethod by using a multi-element antenna. When the user equipmentcommunicates with a first base station while not communicating with asecond base station, the user equipment designs at least one beam havinga main beam directed to a direction of a multipath of the first basestation and having a null directed to the second base station byadjusting a configuration number of the multipath, and forms thedesigned beam as a first-stage beam.

An eighth communication system of the present invention includes a userequipment, and a base station configured to be connected to the userequipment to perform radio communication with the user equipment. Theuser equipment is configured to perform reciprocity-using channelestimation being channel estimation using reciprocity of a channel, foreach frequency band. The user equipment transmits reciprocity capabilityinformation indicating whether or not the reciprocity-using channelestimation can be performed for each frequency band to the base station.The base station performs communication with the user equipment by usingthe reciprocity-using channel estimation in a frequency band in whichboth of the user equipment and the base station are allowed to performthe reciprocity-using channel estimation, based on the reciprocitycapability information of the user equipment.

Effects of the Invention

According to the present invention, satisfactory communication qualitycan be secured.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating the configuration of a radio frame foruse in an LTE communication system.

FIG. 2 is a block diagram showing the overall configuration of an LTEcommunication system 200 under discussion of 3GPP.

FIG. 3 is a block diagram showing the configuration of a user equipment202 shown in FIG. 2 which is a communication terminal according to thepresent invention.

FIG. 4 is a block diagram showing the configuration of a base station203 shown in FIG. 2 which is a base station according to the presentinvention.

FIG. 5 is a block diagram showing the configuration of an MME accordingto the present invention.

FIG. 6 is a flowchart showing an outline from a cell search to an idlestate operation performed by a user equipment (UE) in the LTEcommunication system.

FIG. 7 is a diagram illustrating a method of forming beams in two stagesaccording to a first embodiment.

FIG. 8 is a diagram illustrating another method of forming beams in twostages according to the first embodiment.

FIG. 9 is a sequence diagram illustrating a first example ofre-acquisition at the time of beam disappearance according to the firstembodiment (in a case where the user equipment performs beam detection).

FIG. 10 is a sequence diagram illustrating a second example ofre-acquisition at the time of beam disappearance according to the firstembodiment (in a case where both of the user equipment and the basestation perform beam detection).

FIG. 11 is a sequence diagram illustrating a third example ofre-acquisition at the time of beam disappearance according to the firstembodiment (in a case of dual-connectivity).

FIG. 12 is a diagram illustrating a first example in which nine beamsare formed as first-stage beams according to a second embodiment.

FIG. 13 is a diagram illustrating a second example in which nine beamsare formed as the first-stage beams according to the second embodiment.

FIG. 14 is a diagram illustrating a third example in which nine beamsare formed as the first-stage beams according to the second embodiment.

FIG. 15 is a diagram illustrating an antenna in which a plurality ofbasic elements (dipole antennas or the like) are arrayed in a circularshape according to the second embodiment.

FIG. 16 is a diagram illustrating directivity of beams in two-stagebeamforming according to a third embodiment.

FIG. 17 is a sequence diagram illustrating an example in whichconfiguration of capability of reciprocity for each frequency band isperformed at the time of channel configuration according to a fourthembodiment.

FIG. 18 is a sequence diagram illustrating an example in whichconfiguration of capability of reciprocity for each frequency band isperformed at the time of handover according to the fourth embodiment.

FIG. 19 is the sequence diagram illustrating an example in whichconfiguration of capability of reciprocity for each frequency band isperformed at the time of handover according to the fourth embodiment.

DESCRIPTION OF EMBODIMENTS First Embodiment

FIG. 2 is a block diagram showing an overall configuration of an LTEcommunication system 200 which is under discussion of 3GPP. FIG. 2 isdescribed. A radio access network is referred to as an evolved universalterrestrial radio access network (E-UTRAN) 201. A user equipment device(hereinafter, referred to as a “user equipment (UE)”) 202 that is acommunication terminal device is capable of radio communication with abase station device (hereinafter, referred to as a “base station(E-UTRAN Node B: eNB)”) 203 and transmits and receives signals throughradio communication.

Here, the “communication terminal device” covers not only a userequipment device such as a movable mobile phone terminal device, butalso an unmovable device such as a sensor. In the following description,the “communication terminal device” may be simply referred to as a“communication terminal”.

The E-UTRAN is composed of one or a plurality of base stations 203,provided that a control protocol for the user equipment 202 such as aradio resource control (RRC), and user planes such as a packet dataconvergence protocol (PDCP), radio link control (RLC), medium accesscontrol (MAC), or physical layer (PHY) are terminated in the basestation 203.

The control protocol radio resource control (RRC) between the userequipment 202 and the base station 203 performs broadcast, paging, RRCconnection management, and the like. The states of the base station 203and the user equipment 202 in RRC are classified into RRC_IDLE andRRC_CONNECTED.

In RRC_IDLE, public land mobile network (PLMN) selection, systeminformation (SI) broadcast, paging, cell re-selection, mobility, and thelike are performed. In RRC_CONNECTED, the user equipment has RRCconnection and is capable of transmitting and receiving data to and froma network. In RRC_CONNECTED, for example, handover (HO) and measurementof a neighbor cell are performed.

The base stations 203 are classified into eNBs 207 and Home-eNBs 206.The communication system 200 includes an eNB group 203-1 including aplurality of eNBs 207 and a Home-eNB group 203-2 including a pluralityof Home-eNBs 206. A system, composed of an evolved packet core (EPC)being a core network and an E-UTRAN 201 being a radio access network, isreferred to as an evolved packet system (EPS). The EPC being a corenetwork and the E-UTRAN 201 being a radio access network may becollectively referred to as a “network”.

The eNB 207 is connected to an MME/S-GW unit (hereinafter, also referredto as an “MME unit”) 204 including a mobility management entity (MME), aserving gateway (S-GW), or an MME and an S-GW by means of an S1interface, and control information is communicated between the eNB 207and the MIME unit 204. A plurality of MME units 204 may be connected toone eNB 207. The eNBs 207 are connected to each other by means of an X2interface, and control information is communicated between the eNBs 207.

The Home-eNB 206 is connected to the MME unit 204 by means of an S1interface, and control information is communicated between the Home-eNB206 and the MME unit 204. A plurality of Home-eNBs 206 are connected toone MME unit 204. Or, the Home-eNBs 206 are connected to the MME units204 through a Home-eNB gateway (HeNBGW) 205. The Home-eNB 206 isconnected to the HeNBGW 205 by means of an S1 interface, and the HeNBGW205 is connected to the MME unit 204 by means of an S1 interface.

One or a plurality of Home-eNBs 206 are connected to one HeNBGW 205, andinformation is communicated therebetween through an S1 interface. TheHeNBGW 205 is connected to one or a plurality of MME units 204, andinformation is communicated therebetween through an S1 interface.

The MME units 204 and HeNBGW 205 are entities of higher layer,specifically, higher nodes, and control the connections between the userequipment (UE) 202 and the eNB 207 and the Home-eNB 206 being basestations. The MME units 204 configure an EPC being a core network. Thebase station 203 and the HeNBGW 205 configure the E-UTRAN 201.

Further, the configuration below is studied in 3GPP. The X2 interfacebetween the Home-eNBs 206 is supported. In other words, the Home-eNBs206 are connected to each other by means of an X2 interface, and controlinformation is communicated between the Home-eNBs 206. The HeNBGW 205appears to the MME unit 204 as the Home-eNB 206. The HeNBGW 205 appearsto the Home-eNB 206 as the MME unit 204.

The interfaces between the Home-eNBs 206 and the MME units 204 are thesame, which are the S1 interfaces, in both cases where the Home-eNB 206is connected to the MME unit 204 through the HeNBGW 205 and it isdirectly connected to the MME unit 204.

The base station 203 may configure a single cell or a plurality ofcells. Each cell has a range predetermined as a coverage in which thecell can communicate with the user equipment 202 and have radiocommunication with the user equipment 202 within the coverage. In a casewhere one base station 203 configures a plurality of cells, every cellis configured so as to communicate with the user equipment 202.

FIG. 3 is a block diagram showing the configuration of the userequipment 202 of FIG. 2 that is a communication terminal according tothe present invention. The transmission process of the user equipment202 shown in FIG. 3 is described. First, a transmission data buffer unit303 stores the control data from a protocol processing unit 301 and theuser data from an application unit 302. The data stored in thetransmission data buffer unit 303 is passed to an encoding unit 304, andis subject to an encoding process such as error correction. There mayexist the data output from the transmission data buffer unit 303directly to a modulating unit 305 without the encoding process. The dataencoded by the encoding unit 304 is modulated by the modulating unit305. The modulated data is converted into a baseband signal, and thebaseband signal is output to a frequency converting unit 306 and is thenconverted into a radio transmission frequency. After that, atransmission signal is transmitted from an antenna 307 to the basestation 203.

The user equipment 202 executes the reception process as follows. Theradio signal from the base station 203 is received through the antenna307. The received signal is converted from a radio reception frequencyinto a baseband signal by the frequency converting unit 306 and is thendemodulated by a demodulating unit 308. The demodulated data is passedto a decoding unit 309, and is subject to a decoding process such aserror correction. Among the pieces of decoded data, the control data ispassed to the protocol processing unit 301, and the user data is passedto the application unit 302. A series of processes by the user equipment202 is controlled by a control unit 310. This means that, though notshown in FIG. 3 , the control unit 310 is connected to the individualunits 301 to 309.

FIG. 4 is a block diagram showing the configuration of the base station203 of FIG. 2 that is a base station according to the present invention.The transmission process of the base station 203 shown in FIG. 4 isdescribed. An EPC communication unit 401 performs data transmission andreception between the base station 203 and the EPC (such as the MME unit204), HeNBGW 205, and the like. A communication with another basestation unit 402 performs data transmission and reception to and fromanother base station. The EPC communication unit 401 and thecommunication with another base station unit 402 each transmit andreceive information to and from a protocol processing unit 403. Thecontrol data from the protocol processing unit 403, and the user dataand the control data from the EPC communication unit 401 and thecommunication with another base station unit 402 are stored in atransmission data buffer unit 404.

The data stored in the transmission data buffer unit 404 is passed to anencoding unit 405, and then an encoding process such as error correctionis performed for the data. There may exist the data output from thetransmission data buffer unit 404 directly to a modulating unit 406without the encoding process. The encoded data is modulated by themodulating unit 406. The modulated data is converted into a basebandsignal, and the baseband signal is output to a frequency converting unit407 and is then converted into a radio transmission frequency. Afterthat, a transmission signal is transmitted from an antenna 408 to one ora plurality of user equipments 202.

The reception process of the base station 203 is executed as follows. Aradio signal from one or a plurality of user equipments 202 is receivedthrough the antenna 408. The received signal is converted from a radioreception frequency into a baseband signal by the frequency convertingunit 407, and is then demodulated by a demodulating unit 409. Thedemodulated data is passed to a decoding unit 410 and then subject to adecoding process such as error correction. Among the pieces of decodeddata, the control data is passed to the protocol processing unit 403,the EPC communication unit 401, or the communication with another basestation unit 402, and the user data is passed to the EPC communicationunit 401 and the communication with another base station unit 402. Aseries of processes by the base station 203 is controlled by a controlunit 411. This means that, though not shown in FIG. 4 , the control unit411 is connected to the individual units 401 to 410.

FIG. 5 is a block diagram showing the configuration of the MME accordingto the present invention. FIG. 5 shows the configuration of an MME 204 aincluded in the MME unit 204 shown in FIG. 2 described above. A PDN GWcommunication unit 501 performs data transmission and reception betweenthe MIME 204 a and the PDN GW. A base station communication unit 502performs data transmission and reception between the MME 204 a and thebase station 203 by means of the S1 interface. In a case where the datareceived from the PDN GW is user data, the user data is passed from thePDN GW communication unit 501 to the base station communication unit 502via a user plane communication unit 503 and is then transmitted to oneor a plurality of base stations 203. In a case where the data receivedfrom the base station 203 is user data, the user data is passed from thebase station communication unit 502 to the PDN GW communication unit 501via the user plane communication unit 503 and is then transmitted to thePDN GW.

In a case where the data received from the PDN GW is control data, thecontrol data is passed from the PDN GW communication unit 501 to acontrol plane control unit 505. In a case where the data received fromthe base station 203 is control data, the control data is passed fromthe base station communication unit 502 to the control plane controlunit 505.

A HeNBGW communication unit 504 is provided in a case where the HeNBGW205 is provided, which performs data transmission and reception betweenthe MME 204 a and the HeNBGW 205 by means of the interface (IF)according to an information type. The control data received from theHeNBGW communication unit 504 is passed from the HeNBGW communicationunit 504 to the control plane control unit 505. The processing resultsof the control plane control unit 505 are transmitted to the PDN GW viathe PDN GW communication unit 501. The processing results of the controlplane control unit 505 are transmitted to one or a plurality of basestations 203 by means of the S1 interface via the base stationcommunication unit 502, and are transmitted to one or a plurality ofHeNBGWs 205 via the HeNBGW communication unit 504.

The control plane control unit 505 includes a NAS security unit 505-1,an SAE bearer control unit 505-2, and an idle state mobility managingunit 505-3, and performs an overall process for the control plane. TheNAS security unit 505-1 provides, for example, security of a non-accessstratum (NAS) message. The SAE bearer control unit 505-2 manages, forexample, a system architecture evolution (SAE) bearer. The idle statemobility managing unit 505-3 performs, for example, mobility managementof an idle state (LTE-IDLE state which is merely referred to as idle aswell), generation and control of a paging signal in the idle state,addition, deletion, update, and search of a tracking area of one or aplurality of user equipments 202 being served thereby, and tracking arealist management.

The MME 204 a distributes a paging signal to one or a plurality of basestations 203. In addition, the MME 204 a performs mobility control of anidle state. When the user equipment is in the idle state and an activestate, the MME 204 a manages a list of tracking areas. The MME 204 abegins a paging protocol by transmitting a paging message to the cellbelonging to a tracking area in which the UE is registered. The idlestate mobility managing unit 505-3 may manage the CSG of the Home-eNBs206 to be connected to the MME 204 a, CSG IDs, and a whitelist.

An example of a cell search method in a mobile communication system isdescribed next. FIG. 6 is a flowchart showing an outline from a cellsearch to an idle state operation performed by a communication terminal(UE) in the LTE communication system. When starting a cell search, inStep ST601, the communication terminal synchronizes slot timing andframe timing by a primary synchronization signal (P-SS) and a secondarysynchronization signal (S-SS) transmitted from a neighbor base station.

The P-SS and S-SS are collectively referred to as a synchronizationsignal (SS). Synchronization codes, which correspond one-to-one to PCIsassigned per cell, are assigned to the synchronization signals (SSs).The number of PCIs is currently studied in 504 ways. The 504 ways ofPCIs are used for synchronization, and the PCIs of the synchronizedcells are detected (specified).

In Step ST602, next, the user equipment detects a cell-specificreference signal (CRS) being a reference signal (RS) transmitted fromthe base station per cell and measures the reference signal receivedpower (RSRP). The codes corresponding one-to-one to the PCIS are usedfor the reference signal RS. Separation from another cell is enabled bycorrelation using the code. The code for RS of the cell is derived fromthe PCI specified in Step ST601, so that the RS can be detected and theRS received power can be measured.

In Step ST603, next, the user equipment selects the cell having the bestRS received quality, for example, the cell having the highest RSreceived power, that is, the best cell, from one or more cells that havebeen detected up to Step ST602.

In Step ST604, next, the user equipment receives the PBCH of the bestcell and obtains the BCCH that is the broadcast information. A masterinformation block (MIB) containing the cell configuration information ismapped to the BCCH over the PBCH. Accordingly, the MIB is obtained byobtaining the BCCH through reception of the PBCH. Examples of the MIBinformation include the downlink (DL) system bandwidth (also referred toas a transmission bandwidth configuration (dl-bandwidth)), the number oftransmission antennas, and a system frame number (SFN).

In Step ST605, next, the user equipment receives the DL-SCH of the cellbased on the cell configuration information of the MIB, to therebyobtain a system information block (SIB) 1 of the broadcast informationBCCH. The SIB1 contains the information about the access to the cell,information about cell selection, and scheduling information on anotherSIB (SIBk; k is an integer equal to or greater than two). In addition,the SIB1 contains a tracking area code (TAC).

In Step ST606, next, the communication terminal compares the TAC of theSIB1 received in Step ST605 with the TAC portion of a tracking areaidentity (TAI) in the tracking area list that has already been possessedby the communication terminal. The tracking area list is also referredto as a TAI list. TAI is the identification information for identifyingtracking areas and is composed of a mobile country code (MCC), a mobilenetwork code (MNC), and a tracking area code (TAC). MCC is a countrycode. MNC is a network code. TAC is the code number of a tracking area.

If the result of the comparison of Step ST606 shows that the TACreceived in Step ST605 is identical to the TAC included in the trackingarea list, the user equipment enters an idle state operation in thecell. If the comparison shows that the TAC received in Step ST605 is notincluded in the tracking area list, the communication terminal requiresa core network (EPC) including MME and the like to change a trackingarea through the cell for performing tracking area update (TAU).

The device configuring a core network (hereinafter, also referred to asa “core-network-side device”) updates the tracking area list based on anidentification number (such as UE-ID) of a communication terminaltransmitted from the communication terminal together with a TAU requestsignal. The core-network-side device transmits the updated tracking arealist to the communication terminal. The communication terminal rewrites(updates) the TAC list of the communication terminal based on thereceived tracking area list. After that, the communication terminalenters the idle state operation in the cell.

For example, the following description concerns a technology of changingbasic beams depending on a change in a communication state to maintaincommunication.

The 5G system requires large-volume communication and therefore needs touse a wide bandwidth with high carrier frequencies. However, thisrequires a countermeasure against a propagation loss due to the highcarrier frequencies. In order to compensate for the propagation loss,beamforming with a super multi-element antenna not only in the basestation but also in the user equipment has been studied. With amulti-element antenna, the following two methods have been studied.

One method is a method in which an analog-to-digital converter (AD) anda digital-to-analog converter (DA) are provided for each antenna elementto perform beamforming. It is difficult to secure calculation accuracydue to a low antenna gain. Further, it is known that calculations forenhancing a signal-to-noise ratio (SN ratio) and forming a null of abeam are increased by a cubic order of the number of elements, and areduction in a processing volume needs to be studied in various ways.Note that the SN ratio may also be hereinafter referred to as SN.

For example, a method of forming beams in two stages is known. Accordingto a directivity variable antenna, when the same signal is emitted froman element of each antenna in phase, a signal having narroweddirectivity in a direction perpendicular (right in front) with respectto an emission plane can be sent out. When the phase of the same signalemitted from each element is adjusted to satisfy an expression ofdistance between elements×sin θ, beams having a transmission direction(i.e., directivity) deviated by θ can be formed. In this manner, when aplurality of basic beams having narrowed directivity are formed in thefirst stage to obtain an antenna gain, SN can be enhanced andcalculation accuracy can be enhanced. At the same time, due to {numberof antenna elements}>{number of first-stage beams}, a calculation amountfor forming a null by the first-stage beams can be reduced in the secondstage. Refer to FIG. 7 .

In another method, a plurality of antenna elements are configured in ananalog manner in the first stage to perform desired beamforming. In thiscase, the first-stage analog beams are formed as beams with narrowdirectivity by using a horn antenna or a sector antenna, or making thephase variable in an analog manner. The second stage employs a hybridmethod of digitally forming the first-stage beams, similar to theabove-mentioned first method. Refer to FIG. 8 .

With any of the above-mentioned methods, in a case where beams areformed in two stages by both of the base station and the user equipment,when the user equipment uses first-stage beams (n beams) of the userequipment to receive known sequence data transmitted with first-stagebeams (m beams) of the base station, the user equipment estimates n×mchannels. For example, through diversity and equalization processingperformed with use of the estimated channels, throughput is enhanced.Particularly, in order to remove interference between beams, it isefficient to multiply transmission data of the user equipment by inversecharacteristics of the above-mentioned n×m channels as a precodingweight to form the second-stage beams. For example, the weightcalculation can be done by calculating an inverse matrix of a matrix ofthe n×m channels. When the transmission data is multiplied by theinverse matrix, only the diagonal elements of the matrix remain toeliminate a cross factor between beams. Thus, interference between beamscan be removed.

However, a communication state of the user equipment is changed due tomovement or the like, and accordingly the first-stage beams are changedevery moment. Therefore, there is a problem that channel estimationfails, and correct precoding cannot be performed in the base station.

The first embodiment solves the above-mentioned problem. For example,the following description concerns re-acquisition at the time of beamdisappearance.

Ideally, regarding the first-stage beams of the user equipment, aplurality of first-stage beams cover the whole area. However, whendirectivity is directed in a non-communication direction, directivitycannot be narrowed in a desired direction. Thus, an antenna gain cannotbe obtained. Therefore, it is efficient to orient directivity in adirection of an opposing base station. Alternatively, it is efficient toorient directivity in a direction in which a signal from a base stationdirectly arrives or a direction in which a signal from a base stationarrives through reflection and diffraction. Further, it is efficient todirect the first-stage beams to all the base stations and repeaterstransmitting a primary synchronization signal (PSS) or a secondarysynchronization signal (SSS) other than the base station currently incommunication.

Therefore, the user equipment discontinuously transmits a known sequencesignal corresponding to a signal called sounding, so that the userequipment can monitor variation of the channels of the base station.This signal is transmitted with the directivity of the first-stagebeams. Further, the user equipment may change directivity of the beamswhen the user equipment does not communicate and thereby sequentiallymay monitor neighboring spaces as to if channels have not been changeddue to movement or the like.

For example, when the channels that can be estimated from thefirst-stage beams are significantly changed due to movement of the userequipment or tilting of the user equipment, a received signal of theknown sequence data from the opposing base station disappears or becomesweak. In this manner, the user equipment detects a change of thechannels. When SN is reduced to be smaller than a certain threshold anda change of the channels is detected, if in communication, datacommunication/sounding communication is stopped, and all of the spacesare monitored by using a time period in which data communication isscheduled. When the known sequence data transmitted by the base stationis found, The first-stage beams should be formed to be directed to thebase station and to a sounding signal be transmitted.

If there are a plurality of base station antennas in the same directionand a plurality of channels (links) having a low correlation with aplurality of antennas of the user equipment, and the plurality ofantennas of the user equipment are directed in the direction, thendifferent pieces of data can be simultaneously transmitted and receivedfrom the plurality of antennas, which is efficient.

FIG. 9 illustrates a detailed example of a processing flow.

The user equipment and the base station perform a beam detectionprocedure St901 at the time of initial synchronization establishment.The base station notifies what sort of frequency, timing, and a code (aseed of a spread code or the like) beams for random access are formed,through broadcast information. The user equipment receives the broadcastinformation, and performs neighboring cell/beam search based on thereceived broadcast information. With this, the user equipment monitorsthe status of channel between each beam of the base station and eachbeam of the user equipment, and arrange orders the channel in the orderof high quality.

Next, the user equipment and the base station perform a communicationestablishment procedure St902. The user equipment transmits a channelconfiguration request on a random access channel or the like to anacceptable base station among the base station beams found in St901. Inthis case, the user equipment transmits the channel configurationrequest to the above-mentioned acceptable base station with directivityof the second-stage beams having higher directivity through synthesiswith the first-stages beams of the user equipment. Further, the userequipment transmits the channel configuration request, in accordancewith a resource (frequency, timing, and a code (a seed of a spread codeor the like)) of the base station beams. When the directivity (halfwidth) of the random access is adjusted in accordance with the speed ofthe neighboring monitor cycle of the user equipment, a change in apropagation environment can be handled, and a probability thatcommunication can be established is enhanced. Specifically, it is alsoeffective to employ A×C/D (°) where A (°) is a usual half width otherthan random access, C (ms) is the neighboring monitor cycle of the userequipment, and D (ms) is average time for changing by 3 dB under thespeed of propagation environment change averaged in a moving UE.

A flow of the procedure St903 and later is a flow of a case where thereceived signal of the known sequence data (DMRS or CSI-RS) from theopposing base station disappears or becomes weak due to a change of adirection of the user equipment, for example.

In the procedure St903, the user equipment simultaneously performsnormal communication and measurement of SN of each beam. The userequipment monitors a known sequence of the first-stage beams when theknown sequence is transmitted from the base station with directivity ofthe first-stage beam, and monitors a known sequence of the second-stagebeams when the known sequence is transmitted with directivity of thesecond-stage beams. SN is measured by monitoring the known sequence. Ingeneral, a communication channel is formed with the second-stage beams,and therefore DMRS/CSI-RS transmitted with directivity of thesecond-stage beams is monitored.

In the procedure St904, the user equipment determines whether or not allof the SNs of the second-stage beams used in communication are equal toor less than a certain threshold that can maintain communicationquality.

When a condition that that all of the SNs of the second-stage beams areequal to or less than a threshold is not satisfied, i.e., when at leastone SN of the second-stage beams used in communication is greater thanthe threshold, the user equipment continues the procedure St903.

On the other hand, when all of the SNs of the second-stage beams areequal to or less than the threshold, if the user equipment is incommunication, the user equipment stops data communication/soundingcommunication, and transmits a beam disappearance notification(procedure St905). It is desirable that the beam disappearancenotification be transmitted by using a dedicated/shared channel(PUSCH/PUCCH), for the sake of simplification of communication statetransition. Alternatively, it is important to not disconnectcommunication so that random access may be used for the beamdisappearance notification. Alternatively, it is important not todisconnect communication and also efficient to maximize the half widthor use omni beams as the beams to transmit the beam disappearancenotification, from the viewpoint of directivity (half width) of thebeams.

The user equipment starts a beam re-detection procedure St906 whilewaiting for a response to the beam disappearance notification. Further,it is also efficient to maximize the half width or use omni beams as thebeam to continue data communication with the base station in the middleof St906. Even when a direction of the base station is lost, the omnibeams can be expected to continue communication by reducing atransmission rate.

In a manner described above, when the beam disappearance notification istransmitted from the user equipment, channel quality between beams canbe restored soon.

FIG. 10 illustrates an example in which the base station performs beamdetection, as well as the user equipment. A flow of FIG. 10 is a flowobtained by adding a procedure in the base station to the flow of FIG. 9. The same reference symbols used for the procedure described in theabove is omitted as overlapping description.

The user equipment cannot transmit beams at random when the beams cannotbe identified. Even when the user equipment cannot detect beams for ashort time period, it is also efficient that the base station waits fora while, considering that the channels will be restored soon. Oneexample of this is a case where a truck passes by between the userequipment and the base station. Therefore, similar to the procedureSt903 in the user equipment, the base station measures SN of each beamwhile performing normal communication in St903 b. Then, similar to theprocedure St904 in the user equipment, the base station determineswhether or not all of the SNs of the second-stage beams directed to theuser equipment are equal to or less than a certain threshold that canmaintain communication quality in St904 b.

When all of the SNs of the second-stage beams are equal to or less thanthe threshold, the base station activates a timer to wait for start of abeam re-detection procedure in a procedure St907. When beams equal to orgreater than the threshold cannot be obtained a predetermined number oftimes, the time runs out (refer to a loop of the procedures St908, St903b, St904 b, and St907), and the base station performs the beamre-detection procedure St906.

Note that, in the procedure St904 b, when that condition that all of theSNs of the second-stage beams are equal to or less than the threshold isnot satisfied, i.e., when at least one SN of the second-stage beams isgreater than the threshold, the base station clears the above-mentionedtimer in a procedure St909, and returns to the procedure St903 b.

In a manner described above, even when the beam disappearancenotification St905 cannot be received from the user equipment, the basestation can autonomously start the beam re-detection procedure.

For the sake of simplification, the description above describes anexample in which the time runs out based on the number of times thecondition of {all of SNs of second-stage beams}<{a threshold} issatisfied in St904 b. Instead of this, it is also efficient to employ amethod in which a timer is activated in the base station, and the timeruns out based on the elapse of a predetermined time period. The timeperiod until the time runs out may be defined by a higher layer device,such as operation administration and maintenance (OAM), or may be storedin non-volatile memory as an activation parameter of the base station.

FIG. 11 illustrates an example of yet another processing flow. A flow ofFIG. 11 is a flow obtained by adding a procedure in the MeNB to the flowof FIG. 9 . The same reference symbols used for the procedure describedin the above to omit overlapping description.

FIG. 11 concerns a case where the base station performsdual-connectivity, and only the MeNB handles a shared channel. Accordingto FIG. 11 , when the user equipment determines {all of SNs ofsecond-stage beams}<{a threshold} in the procedure St904, the userequipment transmits the beam disappearance notification to the MeNB, notto an SeNB in communication, in a procedure St1001. In this case, theuser equipment transmits a beam disappearance notification to the MeNBwith omni beams. After the MeNB receives the beam disappearancenotification, the MeNB notifies the SeNB of a beam re-detection commandin a procedure St1002. Consequently, the beam re-detection procedureSt906 is started.

As described above, when the beam disappearance notification to benotified to the MeNB and a beam re-detection command message to betransmitted to the SeNB are provided, even if the user equipment cannotdetect beams, channel quality between beams can be restored soon.

In the examples of FIG. 9 to FIG. 11 here, when a response to the beamdisappearance notification is transmitted from the base station to theuser equipment with the omni beams or the beams of the maximized halfwidth, repeated transmission of the beam disappearance notification fromthe user equipment can be stopped. Consequently, efficiency of radioresources in total can be enhanced

Next, transmission power for transmitting the beam disappearancenotification is described.

In general, when a multi-element antenna is used, narrowing directivityobtains an antenna gain to enable communication even with high carrierfrequencies. In an urban case such as a dense-urban model, it isimportant to narrow directivity so that a transmission signal from theuser equipment does not interfere with other base stations not incommunication and also from the aspect of enhancing communicationcapacity in a system totally.

In such a case, when the beam disappearance notification is transmittedwith omni-directivity in the example of FIG. 9 , the interference can bereduced by configuring the transmission power as follows.

The user equipment transmits a known sequence corresponding to soundingwith directivities of the first-stage beams and the second-stage beams,and transmits a known sequence corresponding to sounding withomni-directivity, in order to perform and maintain communication. Thetransmission may be discontinuously performed in a manner of time,frequency, and a code. The base station performs quality measurement,e.g., SN measurement, of the omni beams, as well as the first-stagebeams and the second-stage beams. Even when SN of the omni beams is low,the omni beams are completely in synchronization with the second-stagebeams in communication. Thus, SN of the omni beams can be accuratelyknown. It is desirable that the base station notify the user equipmentof a result of the SN measurement of the signal transmitted by the userequipment with the omni beams regularly (RRC/PUSCH or PUCCH). When it istime that the user equipment transmits the beam disappearancenotification, the user equipment determines transmission power of theomni beams for transmitting the beam disappearance notification, usingthe SN of the omni beams that is most recently received.

The description above describes an example in which the SN of the omnibeams measured by the base station is notified, but information otherthan the SN may be notified. For example, information of power for theuser equipment to transmit the beam disappearance notification with theomni beams may be notified, in consideration of a change of receptionperformance of a base station, depending on a base station.

For example, it is also efficient to notify by how many more decibelsshould transmission power of the omni beams be increased (or by how manymore decibels may it be decreased) to enable the beam disappearancenotification to reach the base station with SN that can be received bythe base station.

Alternatively, for example, it is also efficient to notify by how manydecibels the omni beams seem lower (or seem higher) than the directivitybeams. The user equipment can determine the transmission power, inconsideration of a modulation level, a coded rate, or the like incommunication, and a modulation level, a coded rate, or the like usedfor the beam disappearance notification.

As described above, the user equipment monitors SN of the base stationin communication, and detects disappearance of a base station signal.When the base station signal disappears, the user equipment notifies thebase station of the disappearance of the base station signal with asignal having wide directivity. Consequently, link re-establishmentbetween the base station and the user equipment can be performed soon,and communication can be maintained.

According to the first embodiment, for example, the followingconfiguration is provided.

A communication system including a user equipment, and a base stationconfigured to be connected to the user equipment to perform radiocommunication with the user equipment is provided. More specifically,the user equipment performs radio communication with a beam. When theuser equipment detects a beam disappearance state being a stateincapable of maintaining communication quality with the base station,the user equipment transmits a notification of the beam disappearancestate with a beam having a wider half width than a half width beforedetection of the beam disappearance state.

Further, a communication system including a user equipment and a basestation configured to be connected to the user equipment to performradio communication with the user equipment is provided. Morespecifically, the user equipment performs radio communication with abeam. When the user equipment detects a beam disappearance state being astate incapable of maintaining communication quality with a first basestation, the user equipment transmits a notification of the beamdisappearance state to a second base station configuring dualconnectivity with the first base station. When the second base stationreceives the notification of the beam disappearance state, the secondbase station gives a command to the first base station to perform beamre-detection processing with the user equipment.

According to such a configuration, the above-mentioned problem issolved, and the above-mentioned effect can be obtained.

Second Embodiment

For example, the second embodiment involves a technology to maintain acommunication by changing basic beams depending on a change in acommunication state, and further involves specifying beams of the userequipment.

When beamforming is performed in two stages by both of the base stationand the user equipment similar to the first embodiment, if the basestation cannot specify beams of the user equipment, throughput isaffected. Specifically, the base station receives known sequence dataform the user equipment, acquires a channel estimation value from thereceived data, and determines beams to be formed based on the channelestimation value. However, when the base station cannot specify thebeams of the user equipment, there is a problem that the base stationcannot determine what sort of beams should be formed to enableimprovement in throughput.

For example, when two beams have a high correlation and thus the twobeams are hardly separated, it is efficient to transmit the same data,instead of transmitting different pieces of data for each beam. This isbecause such a manner of transmission enhances SN and improvesthroughput. On the other hand, if two beams have a low correlation,different pieces of data are transmitted for each beam to gain maximumthroughput. Consequently, actual throughput can be enhanced.

In view of this, a technology of enabling identification of beams havingdifferent directivities and half widths to perform channel estimationfor each beam to gain throughput is illustrated below.

(1) First, the first-stage beams are assigned IDs for enablingidentification of beams having different directions and half widths.

When a null is formed in a desired direction by using side lobes toremove interference between beams as in precoding, IDs that can identifybeams having different directions and peak powers of the side lobes maybe assigned.

Examples in which nine beams are formed as the first-stage beams areillustrated in FIG. 12 to FIG. 14 , and examples of ID assignmentcorresponding to FIG. 12 to FIG. 14 are illustrated below.

TABLE 1 ID assignment example corresponding to FIG. 12 Beam ID 1 2 3 4 56 7 8 9 Angle in horizontal direction (°) −60 −45 −30 −15 0 15 30 45 60Angle in vertical direction (°) 0 0 0 0 0 0 0 0 0 Half width (°) 30 3030 30 30 30 30 30 30

TABLE 2 ID assignment example corresponding to FIG. 13 Beam ID 1 2 3 4 56 7 8 9 Angle in horizontal direction (°) −30 0 30 −15 15 45 −30 0 30Angle in vertical direction (°) −15 −15 −15 0 0 0 15 15 15 Half width(°) 30 30 30 30 30 30 30 30 30

TABLE 3 ID assignment example corresponding to FIG. 14 Beam ID 1 2 3 4 56 7 8 9 Angle in horizontal direction (°) −30 0 0 −15 15 45 −30 0 30Angle in vertical direction (°) −15 −15 −15 0 0 0 15 15 15 Half width(°) 30 30 30 30 30 30 30 30 30

According to the ID assignment examples, it can be known that the beamIDs=4, 5 have the same direction but have different half widths, andtherefore are assigned different IDs.

Further, it can be known that the beams of the beam IDs=2, 3 are emittedto the same area but have different beam IDs. This is effective when anantenna corresponding to the beam ID=2 and an antenna corresponding tothe beam ID=3 are sufficiently away from each other and are in a lowcorrelation. Therefore, it is efficient when there are a large number ofusers in the area or when there is a user using high speed transmission.For example, under a configuration of forming the first-stage beams inan analog manner as in FIG. 8 .

FIG. 12 to FIG. 14 illustrate examples of a relationship betweendirectivity and a half width of a planar antenna, concerning directivityin a range from −90° to +90°. In contrast, configuring directivity and ahalf width in a range from −180° to +180° is also possible, according toa configuration in which a dipole antenna, a helical antenna, or thelike is used as a basic element, and a plurality of basic elements arearrayed in a circular shape (i.e., a cylindrical shape) as in theexample of FIG. 15 , and the first-stage beams are digitally formed.

Here, the user equipment is different from the base station in that theuser equipment significantly changes a direction as time elapses, and inthat surroundings of the user equipment change. Therefore, a situationin which the channels change and are not restored to the original statefrequently occurs. In view of this, it is effective that the userequipment monitors SN of the known sequence signal from the base stationto detect whether or not there is a significant change in the channels,and when it is determined that there has been a significant change inthe channels, different beam IDs are assigned. According to this, misuseof channel information detected from the same beam ID before the changeof the channels can be avoided.

In this manner, new beam IDs are successively assigned. Thus, when pbeam IDs are cyclically used (1, 2, . . . , p, 1, 2, . . . ), p or morebeam IDs can be secured, where p is the number of beam IDs that can beconfigured for the first-stage beams. Consequently, the number ofinformation bits can be limited even when the number of IDs is large.Thus, transmission efficiency can be increased. For example, when thebeam IDs are assigned with zero-based numbering and p=256, aconfiguration of the beam IDs=0 to 255 is employed, and the beam IDs canbe transmitted with 8 bits. The control information is an overhead, andis repeatedly transmitted together with data (U-plane data) used by theuser, and thus reduction in the number of bits even by 1 bit can enhancetransmission efficiency.

The upper limit p of the number of IDs is notified from the base stationby using broadcast information or RRC (corresponding to RRC ConnectionReconfiguration in 3GPP) at the time of channel configuration.

Alternatively, it is also effective to prepare as many IDs as a doublednumber of IDs that can be configured for the first-stage beams (i.e.,prepare two sets of IDs), and use another set of IDs (i.e., toggle thetwo sets of IDs to be used) at the time of changing the beam IDs. TheIDs to be used when to change the beam IDs are notified from the basestation by using broadcast information or RRC (corresponding to RRCConnection Reconfiguration in 3GPP) at the time of channelconfiguration.

(2) Next, means for matching recognition of the beam IDs between theuser equipment and the base station is described.

The first method is a method in which numbers of the beam IDs aretransmitted as data, and in this case, the beam ID data is transmittedwith a beam having directivity and a half width corresponding to thebeam IDs to be transmitted. The beam ID data may accompany the user dataas a control channel (corresponding to PUCCH in 3GPP). The base stationdemodulates the control channel, and when the CRC is okay, the basestation can extract beam IDs of the beam.

The second method is a method in which, unlike transmitting the beam IDsas data, association between the beam IDs and a transmission conditionof the beam IDs (a transmission timing, a transmission frequency, etc.)is configured in RRC in advance. (corresponding to RRC ConnectionReconfiguration in 3GPP), and the base station identifies the beam IDsbased on the beam transmission condition (i.e., based on a detectiontiming, a reception frequency, etc.)

For example, an offset of a transmission cycle of the sounding signaltransmitted by the user equipment with respect to a reference timing ofthe base station is configured in RRC in advance. For example, an offsetnumber for specifying the offset is a symbol number from a head of aframe. Alternatively, when a position to insert the sounding signal isfixed for each slot, a slot number may be used as an offset signal. Anexample is illustrated in a table below. Note that one example of acycle is also illustrated.

TABLE 4 Beam ID 1 2 3 4 5 6 Offset number 5 19 23 37 51 65 Cycle 100 ms100 ms 100 ms 100 ms 100 ms 100 ms

Next, as in OFDM, an example of a case of enabling differenttransmissions per frequency is illustrated. When only one sub-carrier istransmitted and other sub-carriers are not transmitted, transmissionpower of the sub-carrier to be transmitted can be increased to improveSN. Such an example is illustrated in a table below. The frequencynumbers are one example. In view of the fact that a DC offset error canbe reduced by use of a neighbor of the carrier wave when the number ofcarriers to be transmitted is small, the table below illustrates anexample in which the frequency numbers 600 and 601 are used in a case of1200 sub-carriers.

TABLE 5 Beam ID 1 2 3 4 5 6 Offset number 5 19 23 37 51 65 Frequency600-601 600-601 600-601 600-601 600-601 600-601 number Cycle 100 ms 100ms 100 ms 100 ms 100 ms 100 ms

When the first-stage beams are digitally formed, pieces of data havingdifferent directivities can be transmitted at the same time, and thesounding signal can be transmitted in a short time period. Such anexample is illustrated in a table below. Separating the frequencynumbers by 12 is merely one example. An example in which directivity ischanged for each resource block in a case where the resource blockconsists of 12 sub-carriers is illustrated.

TABLE 6 Beam ID 1 2 3 4 5 6 Offset number 5 5 5 5 5 5 Frequency 601-612613-624 625-636 637-648 649-660 661-672 number Cycle 100 ms 100 ms 100ms 100 ms 100 ms 100 ms

It is also efficient to change the transmission cycle depending on thehalf width of the beam. For example, even when the half width isdoubled, total energy is the same as long as the cycle is ½. An examplein which a doubled half width is assigned to the beam ID=1 isillustrated in a table below. The cycle or the half width may bespecified by a frame number, a slot number, or a symbol number, insteadof actual time.

TABLE 7 Beam ID 1 2 3 4 5 6 Offset number 5 19 23 37 51 65 Cycle 50 ms100 ms 100 ms 100 ms 100 ms 100 ms

Further, as for the cycle, it is efficient to configure a short cycledepending on the speed at which neighboring channels change. Therefore,a change speed of the channels may be monitored, and a configurationvalue may be changed depending on a monitoring result. As an index formonitoring the change of the channels, a Doppler frequency measured inthe base station for each user equipment may be used. Alternatively,information about SN [change speed of SN, variability (dispersion) ofSN, or the like] from the opposing device may be acquired in the basestation or the user equipment, and such information may be used as theindex for monitoring the change of the channels. When it is determinedthat a change is needed, re-configuration is performed with RRC(corresponding to RRC Connection Reconfiguration in 3GPP).

The third method is a method in which, as the data to be transmitted, anorthogonal code/quasi-orthogonal code using the beam ID as a seed, suchas a Gold code and an Hadamard code, is used. The sounding signals of aplurality of beams can be simultaneously transmitted, and thus aninformation amount for transmitting the beam IDs being an overhead forthe U-plane data can be reduced. Further, when beam directions aredifferent, the effect of space division multiplex (SDM) can be takeninto consideration for the simultaneously transmitted sounding signals,and SN can be further improved than simply spreading the code.Association between the seed of the orthogonal code/quasi-orthogonalcode and the beam IDs are configured in advance in RRC (corresponding toRRC Connection Reconfiguration in 3GPP), and the base station identifiesthe beam IDs with a detectable seed.

In the above-mentioned first to third methods, to change the beam ICassignment in association with a significant change of the channelsbetween the base station and the user equipment, a method ofre-configuring RRC (corresponding to RRC Connection Reconfiguration in3GPP) at the time of the change or a method of transmitting beam IDs forre-detection in advance is effective.

Alternatively, when IDs for re-detection are transmitted in advance, amessage which has included as many IDs as a multiple of the number ofIDs that can be configured in first-stage beams is transmitted. The userequipment uses different sets of IDs at the time of changing the beamIDs. The base station detects IDs of beams out of a plurality of beamIDs in a blind manner. When the base station detects that the beam IDshave been changed, the base station stops and discards processing ofintegrating channel information detected from previous beam IDs, andcalculates new channel information. An example of the number of IDs thatcan be configured for the first-stage beams=6 and the number of sets ofIDs=2 is illustrated in a table below.

TABLE 8 Beam ID set 1 1 2 3 4 5 6 Beam ID set 2 129 130 131 132 133 134Offset number 5 19 23 37 51 65 Cycle 50 ms 100 ms 100 ms 100 ms 100 ms100 ms

The above-mentioned identification methods of the beam IDs are moreeffective when used in combination.

For example, associating with a transmission timing, a transmissionfrequency and others should be configured for each user equipment in RRCin advance, and an orthogonal code/quasi-orthogonal code using the beamID as a seed, such as a Gold code and an Hadamard code, should be used.According to this, even when a propagation environment is stable, atiming of a beam ID signal and a resource such as a frequency need notbe constantly occupied. Therefore, transmission frequency of the beamIDs of which propagation function needs to be updated along with thevariation of the propagation environment can be selected.

Through the above-mentioned methods, recognition of the beam IDs canmatch between the base station and the user equipment.

Further, through the above-mentioned methods, the base station candetect a change of IDs of beams transmitted by the user equipment.Therefore, it is also efficient to determine beam disappearance when thebase station detects a change of the beam IDs, without transmitting thebeam disappearance notification (refer to St905 and St1001) described inthe first embodiment.

(3) Next, Two-Stage Detection Means of the Beam IDs is Described.

When the beam IDs are transmitted as data, a known sequence common toall of the user equipments and common to all of the beam IDs should bealso simultaneously transmitted. Alternatively, instead of thesimultaneous transmission, the beam ID data and the above-mentionedcommon known sequence should be transmitted by using a resource close tothe extent of not changing the channel (a timing immediately beforetransmitting the beam ID in a radio format or the like). The basestation determines signal quality by using SN or the like to detectwhether or not there is a beam. Only when the base station determinesthat there is a beam, the base station performs beam ID identificationfor the user equipment that the base station allows transmission.Consequently, detection processing of the base station can be reduced.

The known sequence common to all of the user equipments and to all ofthe beam IDs is notified from the base station with broadcastinformation or a medium corresponding to RRC Connection Reconfigurationin 3GPP.

Alternatively, when the beam IDs are transmitted as data, a knownsequence that is different for each user equipment and common to all ofthe beam IDs used by the user equipments should be transmitted. The basestation performs beam ID identification of the user equipment only whenthe base station detects such a known sequence. Consequently, detectionprocessing of the base station can be further reduced.

The known sequence common to all of the beam IDs used by the userequipments is notified from the base station with broadcast informationor a medium corresponding to RRC Connection Reconfiguration in 3GPP.

Further, it is also efficient to sort the user equipments into somegroups, share a resource such as a timing and a frequency within thegroups, and use a group ID for each group. In this case, in order not tooverlap the beam IDs in the user equipments, association between thebeam ID and the resource (a transmission timing, a frequency, etc.) isconfigured in RRC in advance. Although some user equipments need tosuddenly search for many beam IDs again due to dropping and a directionchange, in view of the fact that there are also user equipments such asuser equipments installed in a vending machine that are not assumed tomove, grouping can reduce radio resources due to the statisticalmultiplexing effect.

The known sequence for the group IDs is notified from the base stationwith broadcast information or a medium corresponding to RRC ConnectionReconfiguration in 3GPP.

When two or more, out of the known sequence common to all of those userequipments and common to all of the beam IDs, the known sequence commonto all of the beam IDs used by those user equipments, and the knownsequence for the group IDs, are used in combination, detectionprocessing of the base station can be further reduced.

According to the second embodiment, for example, the followingconfiguration is provided.

A communication system including a user equipment, and a base stationconfigured to be connected to the user equipment to perform radiocommunication with the user equipment is provided. More specifically,the user equipment performs radio communication with a two-stagebeamforming method by using a multi-element antenna. The user equipmenttransmits information for identifying an attribute of each first-stagebeam to the base station.

According to such a configuration, the above-mentioned problem issolved, and the above-mentioned effect can be obtained.

Third Embodiment

For example, the third embodiment concerns a countermeasure againstinterference with another base station.

The first and second embodiments mainly describe beamforming of uplink(the user equipment→the base station). The third embodiment describesbeamforming of the user equipment to improve throughput of downlink (thebase station→the user equipment).

It is known that a method in which the base station acquires downlinkchannel information from a received uplink signal by using reciprocity,and calculates a precoding weight based on the acquired downlink channelinformation, and multiplies downlink transmission data by the calculatedprecoding weight is effective to enhance throughput. However, datatransmitted in such a way does not consider an interference componentfrom another base station, and thus there is a problem that cannot gainan expected throughput.

Solutions to this problem is described below.

The first method is a method in which interference information fromanother base station other than the base station in communication ismeasured by the user equipment, and the information is fed back to thebase station in communication.

The user equipment receives the transmission data multiplied by theprecoding weight and transmitted by the base station in communicationwhile the user equipment reduces interference from another base stationas much as possible by using a degree of freedom of the antenna of theuser equipment with post-coding or the like. However, for example, evenif post-coding or the like is performed, when a degree of freedom of theantenna is insufficient or when a signal (including a reflected wave) ofthe base station in communication and a signal from other base stationsnot in communication come from similar directions, interference from thebase stations not in communication remains.

In view of this, an insufficiency value indicating insufficiency withrespect to required SN, or an interference amount from other basestations not in communication is fed back to the base station incommunication. When the base station in communication increasestransmission power based on the feedback information, throughput can beenhanced. When transmission power is adjusted based on the feed-backinformation for each user equipment in such a manner, total transmissionpower transmitted by the base station in communication can be reduced,as compared to a case where transmission power is uniformly increased inthe base station in communication.

Feedback methods are described.

Information to be fed back is notified to the base station from the userequipment with a medium corresponding to a measurement report of RRC in3GPP. This is different from a measurement report used in neighboringcell monitoring in 3GPP for handover or the like in that it is not areport of reception power of a neighboring base station, specifically inthat an interference amount from other base stations not incommunication is measured with respect to data transmitted by the basestation in communication with beams having appropriate directivity andreceived with the beams having appropriate directivity formed by theuser equipment. When the user equipment detects a PSS and an SSS ofanother base station not in communication, the user equipment stores thedetection timing, frequency, or the like for each cell ID, and measureshow much interference there will be if the signals are received with thebeams having appropriate directivity formed by the user equipmentbetween the user equipment and the base station in communication.

The information to be fed back may be a message element accompanying themeasurement report of RRC in 3GPP.

When precoding processing is calculated through inverse matrixcalculation of a channel matrix, a processing volume is large, and thushigh-performance DSP or a large-scale LSI is required to perform thecalculation in time for fading. In view of this, RRC is effectivebecause an average value of the interference component from other basestations not in communication can be compensated for, althoughreflection cycle may be long.

The description above describes a method of compensating for an averageinterference amount from another base station not in communication, byusing a medium corresponding to the measurement report. In contrast,when the insufficiency value indicating insufficiency with respect torequired SN or the interference amount from other base station not incommunication is fed back to the base station in communication by usingthe PUCCH or the L1 control signal in 3GPP, the feed-back informationcan be performed in a short time period. Therefore, variation of thefading can be compensated for, and more stable communication can beperformed. Further, when a transmission power increase/decrease requestcommand (e.g., 1: 1 dB increase, 0: 1 dB decrease, or +1: increaserequired, 0: change not required, −1: decrease possible) is employed tofeed back the command, it is effective to reduce an information amountof the feedback.

The second method is a method in which adjustment is preformed such thatdata transmission from a plurality of base stations is notsimultaneously performed, by using the information about how muchinterference there will be if receiving signals with the beams havingappropriate directivity formed by the user equipment between the userequipment and the base station in communication (informationcorresponding to a measurement report of RRC in 3GPP).

The base station in communication notifies neighboring base stationshaving the cell IDs notified from the user equipment of informationabout a resource (a timing, a frequency, a spread code, a resourceblock, or the like) of which use is not desired or of which use shouldbe refrained from, through a message between the base stations. Themessage between the base stations may be transmitted via a higher layerdevice of the base station. The base station in communication uses theresource notified in the message between the base stations to transmitdata to the user equipment.

Alternatively, the base station in communication notifies neighboringbase stations having the cell IDs notified from the user equipment ofinformation about a resource (a timing, a frequency, a spread code, aresource block, or the like) of which use is desired in a case ofperforming some transmission, through a message between the basestations. The message between the base stations may be transmitted via ahigher layer device of the base station. The base station incommunication avoids using the resource notified in the message betweenthe base stations as much as possible to transmit data to the userequipment.

The third method is a method which the first-stage beams are formed bythe user equipment as in the following manner to solve theabove-mentioned problem.

Specifically, the user equipment does not transmit a sounding signal asit is with the first-stage beams whose transmission direction and halfwidth are simply configured, but transmits a signal configured in thefollowing steps.

(A1) One first-stage beam (beam with which known sequence data isreceived) is formed so that a main beam is directed to the base stationin communication and that a null is directed to another base stationother than the base station in communication.

As the said other base station other than the base station incommunication, only a base station using reception power having amagnitude of influencing communication or a larger magnitude should beselected. For example, required SN depending on a received modulationscheme, coded rate, or the like is used as a threshold.

(A2) The user equipment, if possible, forms another first-stage beam, inaddition to the beamforming of the above-mentioned (A1), havingdirectivity different from the above-mentioned (A1) so that a null isdirected to other base stations not in communication.

(A3) The user equipment, if possible, forms another first-stage beam, inaddition to the beamforming of the above-mentioned (A1) and (A2), havingdirectivity different from the above-mentioned (A1) and (A2) so that anull is directed to other base stations not in communication.

(A4) Similar to the above-mentioned (A2) and (A3), a first-stage beamhaving different directivity that a null is directed to other basestations not in communication is added.

Alternatively, a signal configured in the following steps may betransmitted.

(B1) The user equipment provisionally determines the number of beams tobe configured in a direction of a multipath from a received signal ofthe base station in communication.

(B2) Beams are formed so that a main beam is directed to the multipathof the above-mentioned (B1) and that a null is directed to other basestations not in communication. When an inverse matrix can be calculated,the beamforming is completed.

(B3) When an inverse matrix cannot be calculated because a direction ofthe main beam and a direction of other base station in communicationcannot be separated, corresponding multipaths are reduced to recalculatethe inverse matrix. In this case, the multipaths are preferentiallyreduced from a direction close to the said base station.

(B4) The above-mentioned (B3) is repeated.

FIG. 16 illustrates an image of antenna formed in the above.

The base station performs precoding as usual.

According to the example of FIG. 16 , the user equipment provides onebeam directed to a base station a as a first-stage beam for transmittinga known sequence (refer to the beam indicated by the broken line). Thebeam directs a null to a base station b. Another beam (refer to the beamindicated by the solid line) directs a null to both the base station aand the base station b. Precoding a signal of the base station to thesetwo beams makes the directivity of the beams including multipaths of thebase station better. In addition, it makes a null orienting to the basestation b so that a configuration in the user equipment does not acceptinterference received from the base station b. As a result of the above,expected throughput can be gained.

When the configuration as described above is employed, interference fromother base stations not in communication can be reduced, and normalcommunication can be performed.

According to the third embodiment, for example, the followingconfiguration is provided.

A communication system including a user equipment, and a base stationconfigured to be connected to the user equipment to perform radiocommunication with the user equipment is provided. More specifically,when the user equipment communicates with a first base station while notcommunicating with a second base station, the user equipment measures adegree that the second base station interferes with a transmissionsignal from the first base station, and transmits a measurement resultto the first base station. The first base station changes transmissionpower of a signal to be transmitted to the user equipment, based on areceived measurement result.

Further, a communication system including a user equipment and a basestation configured to be connected to the user equipment to performradio communication with the user equipment is provided. Morespecifically, when the user equipment communicates with a first basestation while not communicating with a second base station, the firstbase station adjusts a communication condition for interferencesuppression between data transmission from the first base station to theuser equipment and data transmission performed by the second basestation, and requests the second base station to perform datatransmission on the adjusted communication condition.

Further, a communication system including a user equipment, and a basestation configured to be connected to the user equipment to performradio communication with the user equipment is provided. Morespecifically, the user equipment performs radio communication with atwo-stage beamforming method by using a multi-element antenna. When theuser equipment communicates with a first base station while notcommunicating with a second base station, the user equipment forms, asfirst-stage beams, a first beam having a main beam directed to the firstbase station and a null directed to the second base station, and atleast one second beam having a null directed to the second base stationand having directivity different from directivity of the first beam.

Further, a communication system including a user equipment, and a basestation configured to be connected to the user equipment to performradio communication with the user equipment is provided. Morespecifically, the user equipment performs radio communication with atwo-stage beamforming method by using a multi-element antenna. When theuser equipment communicates with a first base station while notcommunicating with a second base station, the user equipment designs atleast one beam having a main beam directed to a direction of a multipathof the first base station and having a null directed to the second basestation by adjusting a configuration number of the multipath, and formsthe designed beam as a first-stage beam.

According to such a configuration, the above-mentioned problem issolved, and the above-mentioned effect can be obtained.

Fourth Embodiment

For example, the fourth embodiment concerns dealing with reciprocitycapability.

When beams are formed with a multi-element antenna, the base stationneeds to be notified of channel information measured for each beam bythe user equipment, and similarly, the user equipment needs to benotified of channel information measured for each beam by the basestation. Therefore, there is a problem that an information amount islarge.

Further, high frequencies are desired for ensuring a continuousfrequency band; however, as the frequency gets high, proportionally afading frequency gets high. In view of this, to follow a change of thechannels, the user equipment and the base station need to have a shorttransmission cycle concerning the channel information notified to eachother.

To solve these, channel estimation using reciprocity of the channels oftransmission and reception has been discussed, with the background oftime division duplex (TDD: transmission and reception with the samefrequency), while, there is a problem in that a device having acapability of reciprocity needs to have the same beam pattern intransmission and reception, and thus implementation is difficult.

In view of this, the fourth embodiment provides a communicationtechnology of enabling configuration of capability of reciprocity foreach frequency band, and describes that high speed communication isenabled in a frequency band having capability of reciprocity even whenthe entire bandwidth does not have capability of reciprocity.

FIG. 17 illustrates a sequence diagram illustrating an example in whichconfiguration of capability of reciprocity for each frequency band isperformed at the time of channel configuration. According to FIG. 17 ,the user equipment transmits reciprocity capability information for eachfrequency band as UE-capability to the base station at the time oflocation registration (Steps St1601 to St1603). In this case, thereciprocity capability information is transmitted with a mediumcorresponding to RRC Connection Setup Complete in 3GPP (corresponding toa response to a channel configuration request or a change request).

At the time of starting connection for data communication or the like(Step St1604), the base station determines reciprocity capability andcapable transmission efficiency for each cell ID and for each frequencyband and starts preparation in the base station. Further, the basestation notifies the user equipment of configuration information ofprecoding capability for each frequency band through a mediumcorresponding to RRC Connection Setup (Step St1606).

For example, when the 2 GHz band is used only for a control signal andwide band transmission of U-plane is performed with a wider bandwidth(such as 15 GHz, 28 GHz, 60 GHz) different from 2 GHz by using amulti-element antenna, 2 GHz does not require reciprocity, and simpletransmitter/receiver is used. Securing reciprocity requires calibrationfor matching beam patterns of transmission and reception, and alsorequires matching of frequency characteristics of transmission andreception. Thus, when reciprocity is not required even in the 2 GHzband, a low-cost user equipment can be achieved.

The base station determines capability of reciprocity based onUE-capability for each frequency band transmitted from the userequipment and capability of the base station itself for each ID. Channelestimation using reciprocity is used only in a frequency bandwidth inwhich both of the base station and the user equipment have capability ofreciprocity. Precoding is also performed using this channel estimationvalue.

As for a frequency in which either of the base station or the userequipment does not have capability of reciprocity, there are options (i)to (iii) shown below.

(i) Reciprocity is not used. The base station gives a command so thatthe user equipment transmits the measured channel estimation value tothe base station, and the base station performs interference removalsuch as precoding.

(ii) Reciprocity is not used. The user equipment calculates what sort ofconfiguration of phase and amplitude for each beam of the base stationreduces interference, based on the channel estimation value measured bythe user equipment. The base station gives a command so that the userequipment transmits the result to the base station, and the base stationperforms precoding based on the acquired information.

(iii) The base station does not perform processing for reducing beaminterference such as precoding.

Configuration information of precoding capability is described below.When precoding is performed by using reciprocity, the base stationspecifies information (a frequency, time (a cycle), a position of aresource block, or the like) for enabling the user equipment to transmitsounding, through a medium corresponding to RRC Connection Setup.

Specifically, in a case of the above-mentioned (i), the base stationspecifies a radio format, a measurement cycle, or the like for reportinga value of downlink channel estimation measured by the user equipment,through a medium corresponding to RRC Connection Setup. The userequipment notifies of the measurement information in accordance with thecommand of the base station, instead of transmitting sounding.

In a case of the above-mentioned (ii), the base station specifies withwhat sort of radio format and cycle the base station makes the userequipment report information of phase and amplitude for each beam of thebase station or index information indicating a combination of phase andamplitude for each beam, through a medium corresponding to RRCConnection Setup. The user equipment notifies of the information inaccordance with the command of the base station, i.e., the informationof phase and amplitude for each beam or the index information about acombination of phase and amplitude for each beam, instead oftransmitting sounding.

In a case of the above-mentioned (iii), the specification as describedabove is not conducted. Alternatively, the base station indicates thatthe configuration of precoding capability is unnecessary, through amedium corresponding to RRC Connection Setup. Transmission in StepSt1609 is not performed. Calculation in Step St1609 is not performed aswell. Therefore, Step St1611, i.e., U-plane data transmission withprecoding, is not performed.

Here, when precoding is performed by using reciprocity, reciprocityrequires different performance depending on transmission efficiency ofinformation. The transmission efficiency of information is determineddepending on a modulation and coding scheme (MCS), the number of capablelayers (the number of streams of signals simultaneously transmitted atthe same frequency), or the like. For example, when the base station hascapability of a scheme of up to 64 QAM and a coded rate of up to ¾, evenif the user equipment has capability of a scheme of up to 256 QAM and acoded rate of up to ⅚, an actual scheme of the user equipment is up to64 QAM and a coded rate of up to ¾.

The base station uses the sounding signal of Step St1609 to calculate aprecoding weight (Step St1610). After the processing, the base stationperforms precoding on downlink data to transmit the downlink data.

Note that, when the base station transmits data before Step St1610, thebase station transmits data without precoding.

When the reciprocity capability information is transmitted asUE-capability of the user equipment from the user equipment to the basestation as described above, even if reciprocity capabilities of the userequipment and the base station coexist, precoding can be performed withcapable transmission efficiency. Particularly, when reciprocitycapability information is provided for each frequency band, even ifreciprocity capabilities coexist for each frequency band in one userequipment, precoding can be performed with capable transmissionefficiency.

FIG. 18 to FIG. 19 illustrate a sequence diagram illustrating an exampleof performing configuration of reciprocity capability for each frequencyband at the time of handover. FIG. 18 and FIG. 19 are connected at theposition of the border line BL. Exchanges of capabilities of a frequencyband that can be used in a destination (destination after handover) areperformed, and the base station determines whether reciprocity up to avalue with lower transmission efficiency is used, or channel estimationis performed without using reciprocity. The determination result isnotified, and then communication of U-plane data is started.

When the user equipment transmits a measurement report (Step St1701) anda source base station on movement determines that communication qualityhas deteriorated worse than a certain threshold based on the measurementreport, a source base station on movement notifies an SGW (a servinggateway) of Handover Required (Step St1702). The SGW notifies the targetbase station on movement of a Handover Request (Step St1703) dependingon the details of the Handover Required. After the SGW receives anotification from the target base station on movement that handover ispossible (Step St1704), the SGW notifies the source base station onmovement that the handover is possible (Step St1705).

Here, the source base station on movement notifies the SGW ofinformation about capability of reciprocity of the user equipment andinformation about a capable MCS and number of layers as information ofthe user equipment (Step St1706). Particularly, it is efficient that theabove-mentioned information to be notified be information for eachfrequency band. The SGW transfers the notified information to the targetbase station on movement (Step ST1707). Therefore, when the source basestation on movement directly transmits a status transfer to the targetbase station on movement, it is efficient because configuration time canbe reduced. The target base station on movement determines capability ofreciprocity, a capable MCS and number of layers, etc. (Step St1708), andnotifies the source base station on movement of configurationinformation of the capability of precoding for each frequency band to beused as a result of the determination (Step St1709).

Subsequent Steps St1710 to St1715 are the same as the example of FIG. 17(example in which configuration of capability of reciprocity for eachfrequency band is performed at the time of channel configuration).

According to the fourth embodiment, for example, the followingconfiguration is provided.

A communication system including a user equipment, and a base stationconfigured to be connected to the user equipment to perform radiocommunication with the user equipment is provided. More specifically,the user equipment is configured to perform reciprocity-using channelestimation being channel estimation using reciprocity of a channel, foreach frequency band. The user equipment transmits reciprocity capabilityinformation indicating whether or not the reciprocity-using channelestimation can be performed for each frequency band to the base station.The base station performs communication with the user equipment by usingthe reciprocity-using channel estimation in a frequency band in whichboth of the user equipment and the base station are allowed to performthe reciprocity-using channel estimation, based on the reciprocitycapability information of the user equipment.

According to such a configuration, the above-mentioned problem issolved, and the above-mentioned effect can be obtained.

Each of the embodiments and modifications of the embodiments describedabove are merely an example of the present invention, and each of theembodiments and modifications of the embodiments described above can befreely combined within the scope of the present invention. Further, anycomponent of each of the embodiments and modifications of theembodiments described above can be changed or omitted as appropriate.

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous unillustratedmodifications can be devised without departing from the scope of theinvention.

EXPLANATION OF REFERENCE SIGNS

-   -   200 communication system, 202 user equipment, 203, 800 base        station

1. A communication system comprising: a user equipment; and a basestation configured to be connected to the user equipment to performradio communication with the user equipment, wherein when the userequipment communicates with a first base station while not communicatingwith a second base station, the user equipment measures a degree thatthe second base station interferes with a transmission signal from thefirst base station, and transmits a measurement result to the first basestation, and the first base station changes transmission power of asignal to be transmitted to the user equipment, based on a receivedmeasurement result.
 2. A communication system comprising: a userequipment; and a base station connected to the user equipment to performradio communication with the user equipment, wherein when the userequipment communicates with a first base station while not communicatingwith a second base station, the first base station adjusts acommunication condition for interference suppression between datatransmission from the first base station to the user equipment and datatransmission performed by the second base station, and requests thesecond base station to perform data transmission on the adjustedcommunication condition.
 3. A communication system comprising: a userequipment; and a base station configured to be connected to the userequipment to perform radio communication with the user equipment,wherein the user equipment is configured to perform reciprocity-usingchannel estimation being channel estimation using reciprocity of achannel, for each frequency band, the user equipment transmitsreciprocity capability information indicating whether or not thereciprocity-using channel estimation can be performed for each frequencyband to the base station, and the base station performs communicationwith the user equipment by using the reciprocity-using channelestimation in a frequency band in which both of the user equipment andthe base station are allowed to perform the reciprocity-using channelestimation, based on the reciprocity capability information of the userequipment.